The invention relates to a method of manufacturing an optical fibre which is provided with a synthetic resin cladding, a curable synthetic resin composition being applied to a glass fibre having at least one enveloping layer of a synthetic rubber, which synthetic resin composition comprises one or more oligomeric compounds whose molecules contain reactive groups and have a molecular weight below 5000, which molecules exhibit a liquid crystalline behaviour, the molecules of the curable synthetic resin composition being oriented during the application of this composition to the glass fibre, after which the curable synthetic resin composition is cured, thereby forming a synthetic resin whose molecules are oriented mainly in the longitudinal direction of the optical fibre.
Glass fibres for optical telecommunication purposes generally have a cladding of a synthetic resin so as to prevent mechanical damage. In order to prevent optical transmission losses as a result of microbends, a cladding is preferred which is built up from various layers. For example, the following method is used. Immediately after the formation of the glass fibre, for example, by drawing from a preform or by means of the double-crucible method, a first soft buffer layer is applied consisting of a synthetic rubber having a modulus of elasticity from 1 to 10 MPa. In order to protect this soft buffer layer during the further processing of the optical fibre, a second harder synthetic resin top layer is applied having a modulous of elasticity exceeding 100 MPa. This top layer is also applied directly after the formation of the glass fibre, for example before the fibre is guided over a pulley or stored. The buffer layer and the top layer together constitute the primary synthetic resin cladding of the glassfibre.
In order to protect the optical fibre from ambient influences during the cabling, during laying the cables and during the life of the cables, the optical fibre is additionally provided with a thicker secondary synthetic resin cladding having a modulus of elasticity exceeding 1 GPa. This secondary synthetic resin cladding is not necessarily applied directly after the formation of the glass fibre.
Two forms of such a secondary synthetic resin cladding are used. In one form the optical fibre with the primary synthetic resin cladding is positioned freely in the secondary synthetic resin cladding which thus forms a tube. The space between the optical fibre and the tube is generally filled with a thixotropic liquid or gel, for example, a silica-filled silicon oil. In the other form the secondary synthetic resin cladding is bonded in an adhering manner with the primary synthetic resin cladding.
It is known that the microbend losses of an optical fibre under transverse load can be made small without resulting in a great temperature sensitivity, by giving the molecules of a part of the synthetic resin cladding a preferred orientation in the longitudinal direction of the optical fibre. As a result of this, the modulus of elasticity of the synthetic resin in the longitudinal direction is increased whereas the coefficient of thermal expansion becomes smaller. The coefficient of thermal expansion of the glass fibre preferably is substantially equal to that of the synthetic resin.
U.S. Application Ser. No. 903,008, filed Sept. 2, 1986 and entitled "Optical Fibre Comprising a Synthetic Resin Cladding and Method of and Device for Manufacturing Such an Optical Fibre" describes the manufacture of such an optical fibre in which the oriented synthetic resin is formed of a curable synthetic resin composition. The molecules of the curable synthetic resin composition are oriented during the application of this composition to the glass fibre, preferably, by means of an elongation flow. In particular a synthetic resin composition is used which comprises one or more compounds which exhibit liquid crystalline properties.